Modulator with emissive diode and photodiode for the modulation of a carrier oscillation with a signal oscillation



FIPBlGb ,NJ 233 EX MaYZL 1968 HANS-NORBERT ToussAlNT ET A1. 3,334,337

A MODJLTCE WITH EMISSE DIODE AND PTC'DIODB. Y

FOR THE MOZULATIOH O? A CHRIER OSCILLIOl! lf?? wma A 51mm, oscILLA'rloN Filed Jan. 21, 1965 Fig] 1 1^ emSSive aphoto f 'i E! 2 UZ diode En I emissive dmde t* n 'I United States effaceur sce 3.384.837 MODULATOR TFH l.\llSSI\'E DiODE ANI) PHOTGDIODE FOR 'iIIE MODULATION OF A CARRIER OSCILLATION WITH A SIGNAL OSClLLATION Hans-Norbert Toussaint, Munich, and Reginlrard Pospischil, Grafeliiug. Germany, assignors to Siemens Ak. ticugesellschaft, Munich, Germany,`a corporation of Gemrany FiIedJan. 2i, 1965, Ser. No. 427,183\`-. Claims priority, application Germany, Jan. 27, 1964, Y

acti-.rms tcl. sst-,3)

ABSTRACT OF THE ISCLOSURE An arrangement for the modulation of a carrier oscilla v tion with a signal os:illation using an emissive diode optically coupled to a photodiode, in which the carrier oscilfl lation and the signal oscillation can be fed as dcsire'd into and in which the modulation product is removed from the photoziicde circuit..

example, in nticrou-'avereceivers In modulators of this type it proves to be relatively difiicult, from a practical standpoint, to completely uhr-.couple individual connections with respect to other connections over a broad frequency range.

In :tnv apparatus according to the invention for the',

modul tion of a carrier oscillation with a signal oscillation, which is provided with means for the frequehcy selective extraction of the required modulation product, a practically complete uncoupl'in'g at least of individual connections i's achieved through the provision of at least one emission diode optically coupled with at least one photodiode in which the emission ldiode is also included in the current circuit of one oi the two oscillations and into the circuit 'of the phctodiode there is inserted the -means for the frequency selective extraction of the re quired modulation product, while in the circuit of the other oscillation there is include'donc uf the two diodes or an additional emission diode optically coupled with a phenomene.

It is advantageous if, instead of a single emission diode, 'several emission diodes are provided and connected in series, the 4radiation of which is fed to the photodiode or diodes. In this connection it is also recommended that, instead of a single photodiode, several photodiodes be provided and electrically connected in parallel.

A further advantageous development resides in the fcature that the circuit is syntmetricaily. dsigned at least at the side of the photodiodes.

In the drawings wherein lilt'ff'fercnce characters indicate like or corresponding parts:

FIG. l illustrates the circuirof a simpleernbodiment of the iru'ention serving for an explanation of the principles involved;

FIG. 2 illustrates aa embodiment of the invention utilizirig 'two oppositely phased electro-optical transducers;

FIG. 3u., illustrates :rn enit-.aliment of the invention util'fing two electro-optical transducers and two optoele .al transducers;

the ernissive diode 'circuit and/or the photodiode circuit.

3,334,837 patented May 2 1, 196s FIG. 3b illustrates a modification of the invention:

FIG. 4 illustrates a further development of the circuit illustrated in FIG.. l; and

FIG. 5 illustrates a further embodiment which pro- 5 ceeds from the circuit arrangement of FIG. 4.

Diode modulators normally employed have a great drawback in that they are not free of-retroaction or feedback. Since the diode is a bi-pole, no uncoupling of the currents 0r voltages of the individuai frequency is possible, and in order to obtain a solution therefor, transistor modulators have been proposed. In these, the modulationeliccting nonlinear emitter-base junction of the transistor t is practically uncoupled from the load disposed in the.

collector circuit of the transistor by a nonreciprocal amplier' element (the transistor). An adequate uncooback in the transistor modulator-can be achieved, however, only at relatively low frequencies, since at higher frequency the" collector circuit is coupled with the input circuit over the collector capacitance and the base re sistance of the transistor. It is the aim of the invention to produce a 'modulatorfree of feedback, even at high modulation frequencies having anorder of magnitude in the gigacycle range. This consists, in its simplest form, of an electro-optical energy transducer and an opt0-electrical transducer optically coupled therewith, inwhich arrangemen'. the load developing the modulation product is oonnectcd electrically in series with the optolectrical transducer. The twosignals to be modulated with one another are fed, either 'additively to the electro-optical transducer, 'or the one signal is fed to the electro-optical transducer an'd the other signal is fed to a series circuit comprising the opto-'eleetrical transducer and th'e load. The modlator according to the invention is free of feedback, because a phototransmission is possible only in the single direction from the electro-optical transducer to the optoelectrical transducer.

FIG. -1 illustrates a rst example of a modulator according to the invention. The ysources of 4th'e twqvoitages' 40 to be modulated with. the frequencies `u1 and u, are

designated as 1 and 2. The voltages of the sources mentioned areadditively applied to the electro-optical transducer 3.. i an emissio-t di gallium-arscnide dime, a gallium-phosphide diodeor the Ve. at is, a. device which transforms the electric currcat owing through it into electro-magnetic radiation and has nonlinear properties. For adjustment to a favorable working point it may, under some circumstances be expedient to insert, in series with the two voltage sources, -a direct voltage source, not illustrated in the basic circuit diagram of FIG. 1.The opto'e'lectrical transducer 4, .for example, a photodiode, is connected i'n series -with the load 5 and a direct voltage source' 6. The term photof diode" is 4intended to mean a device which converts the v electromagnetic radiation into an electric current. The

load 5 in FIG. 1 is, in actual practice, the generally complex input resistance of a filter, by means of which the desired modulation product is separated out. The photons emitted from the electro-optical transducer 3 (indicated in FIG. l by the wavy line 7) strike the opt0-electrical transducer 4, in which they generate electrical charge carriers. I

The operation of the system is explained in the follow The power P, of the light radiated by the eiectro-optical transducer is given by the relation:

P.=NE.t=-hf to I n which N signifies the number of photons entitled per time unit, Eph the energy of thi: photon, h the Planck action quantum and l the frequency ot'the emitted light. According to hypothesis, let there i l' '.-1-...n the volt pling--thut is,.'a,lso an adequate freedom from a f eed,

' n 3,384,837 f i age Us: Urt-.U2 at the elcctroopticttl transducer 3 and the power Ps radiated therefrom. a nonlinear relation ship, which may be described by anexponcntial series.

A Ps=alUs+a2Us2+ A' v 0i special interest as electro-optical transducers are the clectrolutninesccnce plate and the Ga-As emission dinde that has recently become known. In the emission diodes. a distinction is made between two types, the onetype of which emits incoherent radiation whiie the other type emits coherent radiation. The systems according to the invention are capable of operating with both types.

From Equationsl and 2 the nuntber N of photcns emitted per time unit can be deterntinedas a function of The number N of the light quanta emitted per time unit. therefore, is likewise a nonlinear function of the .1`heA opio-electrical transducer 4. forexamplc a photodiode, converts the incident light quanta with the quantum eiliciency: int'o electrical .charge carriers available for 'the current transmission Q=rrN ,(4) Here', Q designates the electric charge carriers generated per time unit' with, in each case, the elementary charge' q. The electrical current IE flowing through the optoelec`- trical transducer is thus given bythe relation i (ai) source 6 for example, a battery. However, since photoresistors allow operating voltages of several hundred volts,

the 4voltage dependency of their quantum e'tciently does' .not constitute any serious drawback.

The quantum c iciency odes, for example of germanium, is approximately one, for example 0.9. The quantum eliicicncy, in the case of adequately high blocking voltages (greater thanone volt.) is practically independent of the magnitude of th'e voltage lying on the pbotodiode. That is, in photodiodes high operating resistances are permissible. A further advantage of the photodiodcs is that they have a low inertia and, therefore', are usable for modulation frequencies in the gigaeycle range.

lt isassumed that the source 1 in the modulator according to FIG. l generates a voltage U eos w1! and the source 2 generates the voltage U, cos ugr. ln Equation 5,.

then, the voltage U, is given by.

U,U cos 1H-U, cos u2! (6) Under the assumption of a voltagealependent quantum cii'tciency q, which can bc fulfilled at least approximately by suitable operating conditions, there is obtained with Equation 5 the current ln flowing in the circuit of the optoelectrical transducer as for semiconductor pl,totodi-" Besides the two current components with the desired frequencies te, 1t-2) and/ur (wp-c2) there arise also other current components with undesired frequencies.-

ln the above equations the devciopmentwas carried out in each case expcit'y only ar to the quadratic lernt. in practice. under some circumstances. terms of higher ordermust also 'ce taken into consideration. This especially applies in the use of cn'nerently radiating emission diodesas electro-opticaltransducers. Front the characteristic-curve equations it is possible in a manner well known for diode mixing stages. aiso to derive the conditions for the operating points and the amplitude relations ot' the individual signas, which' matters. for reasons of brevity, willnot he discus-eed.

Tlh'e components with undesired frequencies can be sttp pressed in a manner known. perse. by suitable formation of the circuit. As an example of this, FIG. 2 illustrates n modulator which uses a second electro-optical transducer 8 fed' in-counterphase. which is likewise optically (pho,- ton current 9) coupled with .the opta-electrical transducer. Y

lt can be sho-.tn that with this circuit arrangement and like working conditions for the. two electrooptical transdueers 3, 8, with reference to the optical-electro transducer 4, currents wtthqthe frequencies u, and u, do not appear in the output circuit.

As a further example of th'e invention, FIG. 3a illusl trates a .modulator with two electro-optical transducers .10, 11. which are optically coupled with the respective opio-electrical transducers 12, 13. The two sources 1, 2

are :so connected over a transformer 14'tha'ton the trans-' ducer 10 there occurs the sum of the two voltages of the sources l and 2,A but at the transducer 11, on the other hand, there occurs the dili'erence of the two voltages of. the'sources 1 and 2, with the optoeleetrical transducers being connected in 'a bridge arrangement. It can be shown that through the load 16, situated in the output circuit of the transformer 15 .there flows currents having the frequenci'cs wg, opt-o2, iai-wz. The battery 11 forms the current ,supplyfor the opt0-electrical transducers. The transformer v15 can be omitted-if the Vload 16 is inserted in the common current lpath of the two optoclectrical transducers, as indicated, for example, in 1:16.35. l FIG. 4 illustrates a further development of the modulator arrangement according to FIG. l, in which use is made of the voltage dependence of the quantum eiliciency" of the optoelcctrical transducer 19. Voltage dependence of the quantum efficiency is here, p urely formally, to be understood as cause for the voltage dependence of the current flowing through the Optoelec-l trical transducer. The physical causes may be of another nature. For example, the drift'velocity of the charge carriers is a. function of the electric eld strength and, thereby-of the voltage being applieth 'llhe source 1 feeds the electro-optical transducer 17,

possibly with inclusion of a direct current source (not represented) serving for biasing the transducer to the.

desired operating point. The photons 18 emitted thereby are supplied to the opt0-electrical transducer 19 which is connected in series with a direct voltage source 20, the

alternating voltage source 2 and the load- 21. If the (apparent)` voltage .dependence of the quantum efiiciency is expressed in the form and there is assumed U,==U, eos all (8b) then thereis obtained the current I! flowing through the Optoelectrical rtansdueer, wherein lcosz 11+ (9) From Equation 9 it is apparent that a modulation occurs even when a linear relation exists bet-wx the n Ain the arrangement of FIG. 4, the selected voltage of the source 2 must be of such a magnitude that the effective voltage at the pho'todiode UD: Url-Um is approximately equal to zero once per cycle of the voltage ot' source 2 or even poles the diode weakly in conducting direction.

The principle explained with the aid of FIG. 4 is also usable in other modulator circuits, for example in pushpull or bridge circuits.

Another example of the invention is illustrated in FIG. 5. which proceeds from the circuit principle of FIG. 4. The signal source 2 is here coupled in the above-described manner, but' with a separate electro-optical transducer 22. The electromagnetic or optical radiation of the transducer 22 Vis supplied to the opt0-electrical transducer 19. So that the required voltage control will be achieved with the signal of 2 in the current circuit of the opt0-electrical transducer 19, a parallel resonance circuit 24, tuned tothe frequencyof 2, is inserted in the transducer circuit..

In closing, consideration should be given to the realization of the optical coupling between the electro-optical andthe opto-electrical transducer. In the simplest case, the

radiation from4 the output .apertureof electro-optical transducers will be allowed to strike the-radiation input aperture of.the Opto-electrical transducer. For the im provement oftlc optical adaptation it is, however, expedient to provide special optical coupling elements. For

this therecome into' consideration: (a)- A collecting lens which is disposcdin the beam path between the electro-optical transducer and the Opto-elec- (c) A fiber optic, such as a socalled optical waveguide, 4

for example of glass liber.

In the event the two optically coupled transducers are spatially arranged too closely, otherwise conditioned by capacitive or inductive coupling of-the two transducers, there may also occur, under some circumstances, an undesired feedback. This feedbaclt can be avoided, as' a practical matter, by greater separation from one another of the two transducers.

Finally, the possibilityialso' exists of carrying out a further modulation in the o'ptical transmission. For this there are suitable, preferably modulators, which utilize the Kerr effect or Pockels effect.

The electrical, series connection of several emission diodesand/or the electrical parallel connection of several photodiodes has, in detail, the following advantages: In

y .th'e series electrical connection of several emission diodes,

4and second signal sources and the diode means are contrical transducer and directs the light, in bundled form,

differential line section has as a consequence a corre' sponding charge carrier movement in each of the emission diodes. ln each of the emission diodes, therefore, through the corresponding simultaneous charge carrier movement there is released from or delivered a proportion of photons corresponding to theparticular quantum efficiency of suchemission diode. Therefore, even if the quantum cfliciency -in the individual. emission diode lies considerably under 100%, through a corresponding number of emission diodes, connected in series, there can be achieved a quarrtum number which lies correspondingly higher, as compared to an individual emission diode of the total number. lt is then possible to arrive at a quantum efficiency greater than 100%, or greater than l.

The photons delivered bythe individual emission diodes can be directly supplied to a common photudiode. lt is then only necessary to correspondingly align the individual emi sion dindes relative to the common photodiode. lu a {ual-er development of the invention, several phnto- 6 diodes, in electrical parallel circuit can be provided for the signal flow. There, for example, a photodiode can be allocated to each emission diode. There may, however, also be provided more or less photodiodes. In' this case it is then only necessary to divide the photon current of the emission diodes correspondingly over the individual photodiodes, electrically connected in parallel for the receptiotrof the signal currents. The use of several photodiodes with an emission diode is recommended, especially ii the emission diode does not bundle its photon radiation into a narrow space sector, but delivers it distributed over -a larger range. It is then possible to appreciably reduce the loss in photons, which otherwise would not reach a. photodiode.

Changes may be made within the scope and spirit of the appended claimswhich dene what is believed to be new and desired to have protected by Lettersv Patent.

1. Apparatus for modulating a first signal with a second signal comprising,

first emissive diode means connected in circuit with the first and second signal sources, 'f a photodiode optically coupled lto the first emissive diode means, and v ltering means connected to the photodiodeto select the desired modulation products. 2. Apparatus according to claim 1 wherein the first nected in series. 3. Apparatus according to claim 2, a second missive diode means connected in parallel with the first emissive diode means and optically coupled to the photodiode.

4. Apparatus according to claim 3 wherein the first and second emissive diodes are poled and are connected in parallel with 'opposite polarity.

5. Modulating apparatus for modulating a first signal with a second signal and removing undesired intermodulaand its other output terminal connected tothe cathode of the other emissive diode,

a transformer with a'primary and two seeondares with itsprimary connected in circuit with the second signal source and with its first secondary having its opposite ends connected to the anodes of the rst and second emissve diodes and the second secondary having its opposite ends connected to-the cathodes of the first and second emissive diodes,

first and second phototliodes with the first diode optically coupled to the first emissive diode and the second diode optically coupled' to 'the second ernissive diode,

the cathode of the first photodiode connected to the anode of the second photodiode,

the anode of .the first photodiode .connected in circuit with the=cathode of the second photodiode,

and an output filtering means connected in circuit with the rst and second photodiodes.

6. Apparatus according to claim S wherein a second.

photodiode and its second 'primary connected across the second photodiode.

7. Apparatus for modulating a first signal with a second signal and removing undesired intermodulatiou products comprising,

first aud second signal sources. an emissive diode means camtected atmssthe first signal source, a photodiode optically coupled to vl esisti-c* diode means and connect source, and

gus-tss? ed in circuit with thc second signal comprising,

first and second signal sources, Y a pair of cmissiye diodes with one connected in circuit with the rst signal source and the other connected in circuit with the second signal source.

a photodiode optically coupledv to the rst and second :missive diodes, and

an output filtering means connected in ci photodiode;

rcuit with the 9. Apparatus according to claim 8 having second filtering means parallel resonant txtl the output frequency of the first signalsource connected in circuit with :the photodiode and the output filtering means.

References Cited- UNITED 3,'1953 1/1951 es/1962 s/wss S/I965 z/rvsv Kk et at'. 33t-f6@ x Lehovcc Z50-499 X Lyman et al. SOT-$8.5

Sack 307-385 Deelman et al. 332-3 X Biard et al 307-885 15 ALFREDv L. BRODY, Primers' Examiner. 

